1. Field of the Invention
The present invention relates to a coated hard metal tool, and more particularly, it relates to a technique of improving the wear resistance of a coated hard metal tool without reducing its breakage resistance, by increasing the thickness of a coating film for a hard metal base material and surface-treating an insert ridge portion for specifying its shape after coating.
2. Description of the Background Art
In general, a tool for cutting a metal material is prepared from cemented carbide, i.e., a WC-Co alloy or an alloy prepared by adding a carbo-nitride of Ti, Ta or Nb. In recent years, however, a hard metal tool prepared by coating a surface of a base material consisting of cemented carbide or cermet or that consisting of alumina or silicon nitride ceramics with a coating film of 3 to 15 .mu.m in thickness by CVD (chemical vapor deposition) or PVD (physical vapor deposition) is increasingly used, in order to attain a high cutting speed. The coating film is prepared from a carbide, a nitride, a carbo-nitride, a carbo-oxide, a boronitride or an oxide of a metal belonging to the group IVa, Va or VIa of the periodic table or Al, or a solid solution thereof. A coating film consisting of diamond or diamond-like carbon is also employed.
Such a coated hard metal tool is prepared by rounding an edge of a hard metal base material by honing, chamfering or composite treatment thereof for forming a cutting edge shape and thereby compensating for the breakage resistance of the breakable hard metal, and then coating the base material with a coating film. The coating film is multi-layered by a well-known technique, in order to improve the wear resistance of the tool. However, it is impossible to sufficiently avoid a problem resulting from such conflicting phenomena that the wear resistance is reduced if the amount of edging of the hard metal base material is increased although the breakage resistance is improved, while the breakage resistance is reduced if the amount of edging is reduced although the wear resistance is improved.
Japanese Patent Laying-Open No. 2-48103 (1990) discloses a technique of partially removing a coating film from an insert ridge portion by surface treatment after coating a surface of a hard metal base material with the coating film thereby improving the strength of a cutting edge and attaining improvement of the wear resistance. In the technique described in the aforementioned patent publication, however, improvement of both breakage resistance and wear resistance may not be necessarily attainable, depending on the degree of removal of the coating film.